Downhole drilling may be performed with many different types of drill bits, including hammer bits that are operated with air or an incompressible fluid, such as water or drilling mud. Air and fluid-driven hammer bits are both effective in some respects, but each type presents several challenges. For example, hammer drilling with air sometimes results in difficulty removing cuttings, and hammer drilling with fluid results in the need to dissipate fluid shocks. In particular, a fluid hammer bit comprises a hydraulically driven percussive drilling tool designed to increase the rate of penetration in hard, friable formations as compared to conventional drill bits, such as roller cones, for example. During drilling, a piston in the fluid hammer cycles continuously between the top of its stroke and the bottom of its stroke when the hammer bit impacts the formation. At these two locations, the hammer piston is not moving, and therefore not consuming any fluid. However, the driving fluid is continuously being supplied to the hammer, such that during those brief moments when the piston is not moving, a fluid shock wave, or pressure pulsation, results. This fluid shock wave is commonly referred to as the “water hammer” effect, which is widely recognized for the potential to cause damage to pipes in any system where valves are suddenly closed, for example. With respect to a fluid hammer, fluid shock waves can be destructive to the hammer itself, to nearby components, and/or to the drill string. These pressure pulsations also represent a loss of hydraulic energy that could be made available to the fluid hammer.
To address such pressure pulsations in other applications, various types of accumulators or pulsation dampeners have been used upstream of devices in hydraulic systems that create pressure pulses. Accumulators are designed to absorb pressure pulses and may also be used to store hydraulic energy. Many hydraulic accumulators are gas loaded and comprise a fluid compartment and a gas compartment with an element separating the two. The fluid compartment communicates with the hydraulic circuit so that as the fluid system pressure rises, fluid enters the fluid compartment of the accumulator, acting against the element, which in turn compresses the gas and stores the fluid in the accumulator. Then, as pressure in the fluid system falls, the compressed gas expands against the element, which in turn forces the stored fluid back into the fluid system. Hydraulic accumulators with separating elements may further be divided into piston-type and bladder-type.
Piston-type accumulators typically comprise an outer cylindrical housing, an end cap at each end of the housing, a piston element, and a sealing system. The housing is designed to hold fluid pressure and guide the piston, which is the separating element between the gas compartment and the fluid compartment. When the gas compartment is charged, the piston is forced against the end cap at the fluid end of the housing. However, when the system fluid pressure exceeds the precharge pressure in the gas compartment, fluid flows in and forces the piston to move in the opposite direction toward the gas end of the housing. Thus, the piston compresses the gas to a higher gas compartment pressure while storing the fluid in the fluid compartment. As fluid pressure inside the accumulator falls below the gas compartment pressure, the gas forces the piston to move toward the fluid end of the housing again and expel fluid from the fluid compartment.
Piston-type accumulators are limited in at least two significant ways. First, the mass of the piston itself slows the response time of the accumulator to pressure spikes or fluctuations in the hydraulic circuit, which is an impediment when the accumulator must respond quickly. Second, the sealing elements disposed between the piston and the housing are exposed to high differential pressures, high velocities, and—in the case of downhole drilling tools—abrasive fluids, and therefore do not have a long operational life.
Bladder-type accumulators typically comprise a pressure vessel and an internal elastomeric bladder that separates the pressure vessel into a gas compartment and a fluid compartment. The gas compartment side of the bladder is charged with an inert gas, such as nitrogen, for example, to a precharge pressure that depends upon the operating pressure of the hydraulic system. The fluid compartment side of the bladder is in fluid communication with the hydraulic system. In the absence of hydraulic system pressure, bladder-type accumulators exposed to high precharge pressures must rely on anti-extrusion devices, such as a plate attached to the bladder, for example, that prevent the bladder from ballooning into the system piping and bursting.
During operation, if the hydraulic system pressure exceeds the gas-precharge pressure, fluid will enter the fluid compartment of the accumulator where that fluid is stored. As the fluid enters, it acts against the bladder, which in turn compresses the gas in the gas compartment until equilibrium is reached between the system pressure and the gas compartment pressure. Any time the hydraulic system pressure rises or falls, the bladder will expand or contract to re-establish pressure equilibrium. For example, if the hydraulic system pressure falls, the gas compartment pressure will also fall when the bladder contracts to force fluid out of the fluid compartment back into the hydraulic system. If the hydraulic system pressure rises, the gas compartment pressure will also rise when fluid flows into the fluid compartment, thereby expanding the bladder to compress the gas in the gas compartment until pressure equilibrium is again reached.
Bladder-type accumulators, although significantly more responsive than piston-type accumulators due to their lower mass, also have some operational limitations. First, some bladder-type accumulators are not inline, meaning the accumulator is not connected axially to the hydraulic system piping. Instead, the accumulator is connected to the hydraulic system from the side. This type of accumulator necessarily requires more radial space than an inline accumulator, which may make it unsuitable for use within a well bore where space is limited. Second, many bladder-type accumulators have anti-extrusion devices that are attached to and move with the bladder, thereby adding mass to the moveable bladder and increasing the response time of the accumulator to pressure fluctuations in the hydraulic system. Third, some bladder-type accumulators have non-moving anti-extrusion devices, such as sleeves with perforations through which the fluid must pass in order to enter or exit the bladder. Such perforations must be small enough to prevent the bladder from extruding in the presence of a precharge pressure that is not counterbalanced by system pressure. However, small perforations limit the response time of the accumulator because the fluid flowing into the bladder must pass through such perforations. In addition, openings like perforations in a sleeve produce turbulence or disturbances in the fluid that can erode the sleeve over time.
Therefore, a need exists for a downhole accumulator designed for high pressures and high flow rates, with an anti-extrusion device that does not significantly inhibit the response time of the bladder by increasing its mass. Moreover, a need exists for an accumulator that is sized appropriately for the space limitations imposed by downhole applications. To minimize costs associated with retrieving the accumulator from the well bore for servicing and repair, a need exists for an accumulator without components that must be frequently replaced due to wear caused by high fluid velocities and high differential pressures.